We went to see the exhibition of Picasso Portraits on the ground floor of the National Portrait Gallery:  a fairly astonishing collection of the full range of Picasso’s work as a painter, draughtsman, cartoonist, amateur filmmaker and, perhaps best of all, sculptor, with a grandly archaic portrait head of Dora Maar, done in 1941, modelled in his Paris bathroom, and sheet-iron portraits of Jacqueline Roque, done in 1962 and now in the National Gallery of Iceland.   What the exhibition proves is that there is no such thing as the evolution of Picasso’s style, but that he uses any and every style as circumstance determined, according to the mood of the moment and how he wants to capture the sitter.